This invention concerns apparatus for performing aperture correction on color signals produced in a color video camera or in a color video signal processor. In particular, it relates to apparatus which monitors the color content of a scene represented by the video signals and automatically selects one or more of the color component signals to serve as the source signal for the aperture correction system.
In video signal processing, aperture correction, also called contour correction or peaking, is a technique by which video signals are processed to emphasize high-frequency components relative to low-frequency components. The effect produced compensates for beam or pixel size (aperture).
High-frequency components in video signals correspond to the edges of objects in the image. A scene containing many small sharply-defined objects is represented by video signals having larger high-frequency components than a scene containing large poorly defined objects. Signal peaking is performed for two reasons: to compensate for aperture distortion in the video camera or video display device and to add extra emphasis to edges in a video display.
The term aperture distortion or aperture rolloff refers to any quality of the video camera or video display device which undesirably increases the minimum size of a pixel. For a video camera, this includes distortion caused by optical elements of the camera, the physical limitations on pixel size caused by the imaging device and any low-pass filtering effect of the electronic components of the camera. Signal processing steps which reduce this type of distortion are often referred to as aperture correction.
Psycho-optic experiments performed in the early days of color television demonstrated that people find images having clearly defined edges to be more pleasing than ones having ill-defined edges. Accordingly, most video cameras and most television receivers in use today allow high-frequency components of the video signals to be boosted in amplitude or "peaked" relative to the lower-frequency components. This type of correction is referred to as peaking or contour correction.
In most video signal standards in use today, the higher-frequency components of the image are represented entirely in the luminance signal. Consequently, aperture correction or peaking is a processing step which is applied to the luminance signal or to each of the primary color signals equally.
Conventional video signal processing systems detect high frequencies in the video signal by monitoring one of the red (R), green (G) and blue (B) primary color signals. Since the green video signal generally contributes the most energy to the luminance signal, it is chosen as the source signal for aperture correction.
A single video signal is selected rather than a combination of two or three of the signals R, G and B, to prevent misregistered portions of the image from being emphasized. If, for example, the source signal to the aperture correction circuitry is a combination of the red signal and the green signal (e.g. a yellow object), and the red image produced by the camera is not precisely registered with the green image, the signal produced by the camera would have both the red edges and the green edges emphasized and, so, would highlight the misregistration of the images. This would appear as a green and/or red border around the yellow object.
One alternative approach for selecting the source signal to be used by aperture correction circuitry is used by SONY in their second-generation HDC300 camera. In this camera, the video operator may select between a combination of the red, green and blue signals; the green and red signals; the green and blue signals; or one of the red, green or blue signals individually as the source signal for the aperture correction circuitry. This choice allows the video operator to selectively peak the video signal according to the requirements of the program and lighting directors.
A problem with either of these systems is that if a single color signal is used as the source signal for the peaking circuitry, then edges of colored objects which are not the selected color are not emphasized. If, however, multiple color signals are selected as the source signal, then misregistration errors are emphasized.